Generators for use in aerospace applications are high output, high speed devices with critical weight and volume restrictions. Such devices typically have permanent magnet rotors, which have a plurality of rare earth permanent magnet elements located on a shaft of magnetizable material. The shaft has a number of flat machined faces about its periphery, onto which the rare earth permanent magnet elements are mounted.
Spacers of aluminum or some other non-magnetizable material are located around the periphery of the shaft in the areas between the magnets, and rings of the same type of non-magnetizable material are located on both ends of the shaft axially adjacent the annulus of permanent magnets and spacers. To prevent the magnets from moving radially outward during the high-speed rotation of the rotor, an outer sleeve also made of non-magnetizable material is shrink fitted over the magnets, spacers, and rings.
Such construction, while producing acceptable generators for most aerospace applications, presents several major problems resulting in reduced performance, a high reject rate, and a relatively high unit cost. The most critical performance problem is that of heat buildup in the rotor. Even if the spacers between the magnets and the rings adjacent the ends of the magnets are machined very carefully, the heat transfer characteristics of the rotor are not uniform, thus leading to heat buildup which may possibly result in damage to the rare earth permanent magnets. A second, and closely related, problem is that of machining the spacers and rings to fit properly. Since the rotor must be stable at very high rotational speeds, if the rings and spacers do not fit exactly, thus allowing relative movement of the components under the outer sleeve, the result will be an unbalanced condition in the rotor possibly resulting in dynamic failure of the device.
A further problem encountered in the construction of such a permanent magnet rotor is that the rotor assembly does not have good rigidity or shaft stiffness, which will in turn reduce the flexure critical speed of the device, the maximum speed of rotation without significant dynamic vibration occurring. Since the rotor must be turned at a very high rate of speed, the lack of proper stiffness in the shaft construction will result in a high rejection rate at best, and possibly in a product which will not perform within the required specifications. Finally, construction of the permanent magnet rotor with a shaft, magnets, spacers and rings, and the outer sleeve is an extremely expensive method of manufacture. The very precise tolerance requirements of the spacers and rings and the high unit rejection rate both add further to the high cost of construction of such rotors.
In certain aerospace applications, it is desirable to have more than one set of magnets located on a single shaft. In such cases, the shaft is relatively long and contains a plurality of sets of permanent magnets spaced axially away from each other on the shaft. For example, a shaft having three sets of magnets would have sequentially located on the shaft a ring, a set of magnets and spacers, a second ring, a second set of magnets and spacers, a third ring, a third set of magnets and spacers, and a fourth ring. Such an assembly does not have sufficient shaft stiffness to allow the shaft to be turned at the required high speed. Long before the shaft reaches the desired operation speed, the flexure critical speed will be reached and dynamic failure will occur.
Therefore, it can be seen that a new type of construction for such high-speed, permanent magnet rotor machines is required. Any new construction technique must minimize heat buildup in the rotor, eliminate the problem of improper fit between the spacers, magnets, and rings, and ensure that the shaft has sufficient rigidity while maintaining or improving the cost characteristic of the device.